It is well known that as the expansion ratio of an internal combustion engine is increased, more energy is extracted from the combustion gases and the thermodynamic efficiency increases. It is further understood that increasing compression increases both power and fuel economy due to further thermodynamic improvements. The objectives for an efficient engine are to provide high compression, begin combustion at maximum compression and then expand the gases as far as possible against a piston.
Conventional engines have the same compression and expansion ratios, the former being limited by the octane rating of the fuel. Furthermore, since in these engines the exploded gases can only be expanded to their initial volume, there is usually a pressure of 70-100 psi against the piston at the time the exhaust valve opens with the resultant loss of energy.
Many attempts have been made to extend the expansion process in internal combustion engines to increase their thermodynamic efficiency. An early design was described in the Brayton cycle engine of 1872 (U.S. Pat. No. 125,166). This engine expanded the combustion gases to their initial pressure but lacked the means of transferring and igniting the charge while maintaining maximum compression. The Atkinson cycle engine was devised to extend the expansion process, but this engine was limited by its mechanical complexity to a one-cylinder configuration.
A notable attempt was more recently revealed in the Wishart engine, disclosed in U.S. Pat. No. 3,408,811, in which a large piston compressed the charge into a smaller cylinder which further compressed the charge and then transferred it into another small "firing" cylinder where the charge was ignited and expanded to the full volume of the smaller cylinder. It then passed the burned gases through ports uncovered by the piston into a larger cylinder where it was expanded further. This required four cylinders with pistons which made two working strokes for each power stroke, hence it is an eight-stroke cycle engine with all of the mechanical and fluid friction inherent in such a working cycle. The mechanical complexity of this engine makes it costly to manufacture.
In another attempt (Vivian, U.S. Pat. No, 4,174,683), the induction valve in the working cylinder of the engine is kept open during part of the compression stroke and thereafter closing the valve and compressing only a fraction of a full charge which is then ignited and expanded against the piston to the full volume of the cylinder. This process is very complex requiring means for both changing the point of axis of the crankshaft and for altering the intake valve timing according to load demands. Furthermore, no means of increasing compression or charge turbulence is provided. This concept continues to operate with the friction inherent in the four-stroke cycle engine. In addition, the operation of this engine at full load is the same as for a conventional engine so that it offers improved characteristics at part load only.
Others have attempted to extract more shaft work from combustion gases using similar systems of conducting the burned gases into other cylinders after firing for additional expansion, also with similar results. Some have tried burning charges in one-half the cylinders of a multi-cylinder engine and then ducting the exhaust from the firing cylinders into the remaining half of the cylinders for the extraction of additional shaft work. To date none of these attempts has been successful and emissions were generally increased over conventional engines.
Rotary engines have also been patented which strive to gain the same advantages. One such is the new Wankel engine, U.S. Pat. No. 3,688,749 issued in 1972, in which a charge is compressed in one chamber of the rotor of a four-lobed rotor engine where the charge is ignited and expanded first in the initial chamber and then through a duct into the next down-stream chamber. Some of the problems with this concept are that the second expansion chamber is already half filled with recompressed exhausted gases from the previous firing and there are extensive throttling losses in transferring the charges.